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Middleborns disadvantaged? testing birth-order effects on fitness in pre-industrial finns

Faurie, C., Russell, A.F. and Lummaa, V. (2009) Middleborns disadvantaged? testing birth-order effects on fitness in pre-industrial finns. Plos One, 4 (5). Art No.e5680. ISSN 1932-6203

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Parental investment is a limited resource for which offspring compete in order to increase their own survival and reproductive success. However, parents might be selected to influence the outcome of sibling competition through differential investment. While evidence for this is widespread in egg-laying species, whether or not this may also be the case in viviparous species is more difficult to determine. We use pre-industrial Finns as our model system and an equal investment model as our null hypothesis, which predicts that (all else being equal) middleborns should be disadvantaged through competition. We found no overall evidence to suggest that middleborns in a family are disadvantaged in terms of their survival, age at first reproduction or lifetime reproductive success. However, when considering birth-order only among same-sexed siblings, first-, middle-and lastborn sons significantly differed in the number of offspring they were able to rear to adulthood, although there was no similar effect among females. Middleborn sons appeared to produce significantly less offspring than first-or lastborn sons, but they did not significantly differ from lastborn sons in the number of offspring reared to adulthood. Our results thus show that taking sex differences into account is important when modelling birth-order effects. We found clear evidence of firstborn sons being advantaged over other sons in the family, and over firstborn daughters. Therefore, our results suggest that parents invest differentially in their offspring in order to both preferentially favour particular offspring or reduce offspring inequalities arising from sibling competition.

Item Type: Article
Copyright, Publisher and Additional Information: © 2009 Faurie et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Institution: The University of Sheffield
Academic Units: The University of Sheffield > Faculty of Science (Sheffield) > School of Biological Sciences (Sheffield) > Department of Animal and Plant Sciences (Sheffield)
Depositing User: Miss Anthea Tucker
Date Deposited: 20 Jan 2010 11:12
Last Modified: 09 Jun 2014 04:26
Published Version: http://dx.doi.org/10.1371/journal.pone.0005680
Status: Published
Publisher: Public Library Science
Refereed: Yes
Identification Number: 10.1371/journal.pone.0005680
URI: http://eprints.whiterose.ac.uk/id/eprint/10316

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